The use of the name Belgian for the language is to some extent supported by Caesar's De Bello Gallico which mentions that the Belgae and the Galli spoke different languages. It is furthermore supported by toponyms in present-day Belgium which, according to the linguist Hans Kuhn, point at the existence of an Indo-European language distinct from the Celtic and Germanic languages.[10]

Proponents of the Belgian language hypothesis also suggest that it was influenced by Germanic languages during a first, early "germanicization" in the third century BCE – as distinct from the Frankish colonization in the fifth to the eighth century. For example, the Germanic sound shifts (p → f, t → th, k → h, ǒ → ă) have affected toponyms which supposedly have a Belgian-language origin.

Characteristic of Belgian said to include the retainment of p after the sound shifts. Names of bodies of water ending in -ara (as in the name for the Dender), -ănā or -ǒnā as in Matrǒnā (nowadays Mater) and settlement names ending in -iǒm are supposedly typically Belgian as well.

According to Gysseling, traces of Belgian are still visible. The diminutivesuffix -ika, the feminizing suffixes -agjōn and -astrjō and the collective suffix -itja- have been incorporated in Dutch, sometimes very productively. In toponymy, apa, poel, broek, gaver, drecht, laar and ham are retained as Belgian loanwords.